During oil and gas well drilling operations, it is often necessary to seal the space between the outer casing string, or pipe and the rock formation through which the drill has passed in order to prevent the undesired flow of fluids including gas, oil and incompressible media such as water. Undesirable fluid flow can be in an upward direction either along the well pipe casing or downwardly where it might be dissipated into the formations below and/or mixed with other fluids.
A variety of compositions are pumped into the bore hole adjacent to the rock formation for the purpose of providing a tight seal between the metal well casing pipe and the surrounding irregular rock surface. The cement composition is disposed in the form of highly flowable slurry that will easily pass downwardly through the irregular annulus formed by the casing and surrounding rock. Its particular formulation will depend upon a number of parameters related to the local conditions. Various mechanical devices are available to provide a mechanical block at the lower-most portion of the bore hole in order to stop the flow of the cement slurry.
The cementing can be required at considerable depths so that the slurry composition is subjected to extremes of high pressure and temperature as it is curing and forming a monolithic mass in the irregular annulus. It is well known in the art that, under these conditions, various cement compositions will either shrink or expand to some extent. Since the cement compositions are often formulated at the site of the well in order to accommodate the specific subterranean conditions present and the downhole rock formations, it is desirable to know in advance how a particular “custom” cement composition will perform with respect to expansion/shrinkage under the subterranean conditions of setting and curing.
Test apparatus for cement samples vary in their purpose and methods. Cement testing apparatus that use pistons to directly pressurize and measure the changes in test samples can introduce errors into measuring test sample shrinkage. These errors occur when portions of the cement test sample build up between the edges of the piston and vessel assembly wall and prevent or slow the downward movement of the piston. This can result in an inaccurate pressurization and measurement of the shrinkage by the pressurizing piston during the test.
It is therefore a principal object of the invention to provide a laboratory test method of and an apparatus for measuring the expansion and/or shrinkage of cement slurry as it sets and cures under pressure and temperature conditions simulating those in the well in which the particular cement composition is to be utilized.
It is another object of the invention to provide a test apparatus that is reliable, sturdy and readily reused.
It is a further object of the invention to provide a pressure and temperature test apparatus that uses a flexible diaphragm to isolate the test sample and an incompressible medium for pressurizing the test sample to overcome the likelihood of introducing errors into the measurement of the changes in the test sample.
It is still another object of the present invention to measure shrinkage and expansion of the test sample by the change in volume of the test sample.
It is another object of the present invention to provide a floating piston and a measuring device that are pressure balanced to the pressure applied to the test sample such that when the volume of the test sample changes, the piston changes position and the measuring device determines the amount of the volume change of the test sample by the change of position of the piston.
Yet another object of the invention is to provide a laboratory test method and apparatus that is capable of measuring both expansion and shrinkage of a cement composition sample for which there is no previous data as to whether the cement composition is likely to shrink or expand upon curing.